1. Field of the Present Invention
The present invention relates to a projection system comprising a projection screen, a projector, and an external light screen.
2. Description of the Related Art
A conventional projection system is generally designed to display an incident image light, from a projector, on a projection screen, and an observer observes its reflected light as an image.
Recently, along with downsizing, price-reduction, etc. of the projector main body, demand for use at home, such as home theater system, is increasing, and the projection systems are widely used in general household. In such projection system, image contrast of an image is brought forth by intensity difference of the incident light (image light), from the projector, incident on the projection screen. For example, when displaying a white picture on a black background, the portion of the projection screen lighten by the incident light will be white and the other portion will be black, thereby the image contrast is brought forth by such difference in brightness between black and white. Therefore, to realize a favorable image display, it is required to emphasize the contrast difference by making the white display portion brighter and the black display portion darker.
However, when such projection system is used at home, for example, it is often placed in a living room, and such place is usually designed to get ambient light such as external light. Accordingly, the projection system reflects ambient light, such as external light, as well as the image light, and both white display portion and black display portion become brighter so that the difference of brightness between black and white becomes smaller. As a result, in such conventional projection screen, it is difficult to realize a favorable image display unless influence by the ambient light such as external light is suppressed by using a means to darken the room or other environmental means.
To solve such problem, it has been studied to develop a projection screen capable of realizing favorable image display even in the presence of high levels of ambient light. For example, those using hologram are proposed. The projection screen using hologram has advantages that it can make the white display portion brighter by controlling the scattering effect, it realizes a relatively favorable image display in presence of high levels of ambient light, and it has wavelength selectivity. However, it does not have polarized light selectivity, and there is a problem that it can only display an image with sharpness of limited degree. Further, there is a problem that, in the projection screen using hologram, it is difficult to obtain a wide screen because of manufacturing problem.
On the other hand, as a projection screen capable of realizing favorable image display even in the presence of high levels of ambient light, a projection screen using a polarized light selective-separation layer is proposed (see Japanese Patent Application Laid-Open Nos. 5-107660 and 2002-540445). The projection screen using polarized light selective-separation layer is capable of making the black display portion darker while keeping the white display portion bright, and as compared with ones using hologram, a sharp image can be displayed in the presence of high levels of ambient light. Specifically, a cholesteric liquid crystal, which reflects right-handed circularly polarized light or left-handed circularly polarized light contained in image light, can be listed. For example, when a cholesteric liquid crystal, which reflects right-handed circularly polarized light, is used, the left-handed circularly polarized light is not reflected by the circularly polarized light separation function of the cholesteric liquid crystal, and influence of the ambient light can be reduced.
However, when using, for example, a cholesteric liquid crystal for reflecting right-handed circularly polarized light, since the right-handed circularly polarized light of environmental light is reflected, sufficient improvement of the contrast cannot be realized in some cases.
Supposing a light source in natural field, almost all of the light derives from the sunlight, and as the ambient light proceeding into the projection system from the external light entrance, there are direct light from the sun and reflected light of which the direct light is reflected by some object. Usually, the direct light from the sun can be separated into, to the proceeding direction, P waves (longitudinal waves) and S waves (transversal waves). Considering reflection of the light On a dielectric interface, while reflection is within Brewster's angle, S waves are totally reflected, and hence the reflected light usually contains a lot of S wave components. Specifically, if the outer field is wet by rain, etc., a lot of S wave component is contained in the reflected light, due to the total reflection at air-water interface. When such reflected light containing much S wave components is seen from inside of the projection system, so-called glare occurs, and visibility is not satisfactory.